go env and go help environment show the variables that matter to building Go programs.
The ones listed here matter to running Go programs and are documented in 'go doc runtime'.
We should not confuse matters by expanding the go command to document things that don't have to do with the go command.
I think things are already quite confusing. For example, GODEBUG affects both go command and runtime. It is shown in go help environment output like that:
Enable various debugging facilities. See 'go doc runtime'
So it mentions runtime, but not how it affects go command. Additionally, even setting GODEBUG to something that affects go command (for example, export GODEBUG=gocacheverify=1) does not add it to go env output, and go env GODEBUG prints nothing.